A simple method for the in situ analysis of femtosecond UV pulses in the pump-probe spectroscopy of solutions

106

Two-photon absorption ͑2PA͒ spectroscopy in the range from 7 to 10 eV provides new insight on the electronic structure of liquid water. Continuous 2PA spectra are obtained via the pump-probe technique, using broadband probe pulses to record the absorption at many wavelengths simultaneously. A preresonance enhancement of the absolute 2PA cross section is observed when the pump-photon energy increases from 4.6 to 6.2 eV. The absorption cross section also depends on the relative polarization of the pump and probe photons. The variation of the polarization ratio across the spectrum reveals a detailed picture of the 2PA and indicates that at least four different transitions play a role below 10 eV. Theoretical polarization ratios for the isolated molecule illustrate the value of the experimental polarization measurement in deciphering the 2PA spectrum and provide the framework for a simple simulation of the liquid spectrum. A more comprehensive model goes beyond the isolated molecule picture and connects the 2PA spectrum with previous one-photon absorption, photoelectron, and x-ray absorption spectroscopy measurements of liquid water. Previously unresolved, overlapping transitions are assigned for the first time. Finally, the electronic character of the vertical excited states is related to the energy-dependent ionization mechanism of liquid water.

Section: B Experimental Two-photon Absorption Spectrum and Polarizatsupporting

confidence: 56%

Section: Introductionmentioning

confidence: 99%

Electronic structure of liquid water from polarization-dependent two-photon absorption spectroscopy

Elles¹,

Rivera²,

Zhang³

et al. 2009

106

“…The most likely contribution is two-photon absorption as reported by Laubereau; however, the intensity of this absorption should reflect the two-photon spectrum and thus the spike height at 800 nm should be extremely weak. 57 Looking at pure water, the transient signal around time zero scaled linearly with pump power. The intensity of the transient is reduced with the pump and probe perpendicularly polarized, but its presence stills leads to some uncertainty in fitting a rise to the electron appearance.…”

Section: Resultsmentioning

confidence: 99%

The ejection distribution of solvated electrons generated by the one-photon photodetachment of aqueous I− and two-photon ionization of the solvent

Kloepfer

Vilchiz

Lenchenkov

et al. 2000

194

362

Articles you may be interested inOne-photon photodetachment of I − in glycerol: Spectra and yield of solvated electrons in the temperature range 329 T 536 K Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na − in THF The spectra and the relative yield of solvated electrons produced by resonant photodetachment of iodide anion in ethylene glycol in the temperature range 296T453 K Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopyThe ultrafast dynamics following one-photon UV photodetachment of I Ϫ ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump-probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ϳ15 Å to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the ''ejected'' electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due to an electron in a contact pair. The contact pair is surprisingly long-lived and the nonadiabatic recombination is rather slow considering the proximity of the partners. Experiments in mixed solvents confirm our assignment of the two distinct ejection mechanisms. The detachment mechanism is therefore fundamentally different in the resonant ͑one photon͒ charge-transfer-to-solvent ͑CTTS͒ process from the multiphoton detachment of aqueous iodide ions, which bears more similarity to the direct solvent ionization.

“…All A signals were corrected for the group velocity dispersion of the probe light with the 25 fs accuracy, utilizing the position of the instrument response functions (IRF), namely, Gaussian-shaped A signals observed in neat solvents as a result of the simultaneous absorption of a pump and a probe photon when the pump and probe pulses temporally overlap. 73,74 The IRF width (fwhm) increases with an increase in probe photon energy from 139 fs in the visible range to 148 fs at 380 nm, 221 fs at 280 nm, and 387 fs at 214 nm in methylcyclohexane. In acetonitrile, the IRF widths were smaller by a factor of 1.26 at wavelengths shorter than 380 nm.…”

Section: B Ultrafast Transient Absorptionmentioning

confidence: 99%

Global sampling of the photochemical reaction paths of bromoform by ultrafast deep-UV through near-IR transient absorption and ab initio multiconfigurational calculations

Pal

Mereshchenko

Butaeva

et al. 2013

Ultrafast deep-ultraviolet through near infrared (210-950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr3 leads to the ground-state iso-CHBr3 product in a large quantum yield (∼35%), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr3 and the ground electronic state of iso-CHBr3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C-Br-Br bending and Br-Br stretching modes of ground-state iso-CHBr3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water.